I have stock 17 in. bullitts on my gt right now and i wanna get deep dish on the back. Besides the cost of tires are there any downsides or issues with having 18 in wheels on the back and 17 in on the front?

Oh i thought your just unbolted that OEM bracket and installed the new one.

Yea I figured you didn't that's why I clarified it and for anyone else that didn't know. I looked into it I just can't take a grinder to my car just for some smaller tires lol. If I was racing it I would but I'm not.